Resin is the first ingredient of the plastic material before the addition of other ingredients like stabilizers, filler, pigments, catalysts, and lubricants, and before fabrication to finishing articles.
First of all, we prepare the polymer and this polymer we add
These are known as curing agents. Thus the resin is actually the first material that is prepared from a polymer.
Types of resins
There are two types of resins
- Thermoplastic resin
- Thermosetting resin
These are those which can be changed into other forms under the action of heat and pressure, the process is reversible. For example, polyethylene, polypropylene, and polyvinyl chloride (PVC)are examples of thermoplastic resins.
These are those which after the formation can not be changed into other forms under the action of heat and pressure these changes are irreversible e.g.
- Phenol formaldehyde resins
- Urea-formaldehyde resins
- Melamine resins
- Acrylic resins
- Polyester resins
Phenol formaldehyde resin
The material in which high temperature and heat are involved, and with high insulating properties, is called formaldehyde material. It is available in the commercial name formatin, which contains 30~35 % formaldehyde and the rest is water. para-formaldehyde is the evaporation of formalin which contain 90% formaldehyde and 10% rest is water.
Preparation of Phenolic resin
We prepare formaldehyde resins and then the compound is obtained by a different method. The following reaction takes place during the formation of formaldehyde resins.
-CH2– is also called methylene bridge
On further condensation we get bakelite
Further methylene bridges are ground into small pieces to obtain the molding powder then the following material is added
- Resins 30Kg (molding powder)
- Filler 40~45 kg(usually wood flour)
- Hexamine ( as a gardening agent)
- MgO 1~2 kg(as a catalyst) CaO can also be used
- Colouring material4~5 Kg
- Lubricating agent ( steric acid) 0.1~0.2 kg
- These are high-temperature resistance for flame and electricity.
- These have high surface hardness, electric insulating properties
- they are resistant to alkalies, acids, and other chemicals
- Also, they are low-cost resins
Uses phenolic resins
These are used for making molding powder used for making plywood.
These are also used as foaming materials high-quality foams with remarkable properties are produced by phenolic resins. Moreover, this kind of foam is fire resistant having good thermal insulating properties.
- These are used for surface coating e.g.; metals are corroded in the presence of oxygen and water.
- phenolic resins are also used to prevent these metals from corrosion.
- These are also used in the plastic and paper industries.
- It is used for preparing chip board i.e. order to improve mechanical properties and to reduce water absorption, phenolic resins are used
Types of phenolic resins
There are two types of phenolic resins
- Novolac resins
- Resol resins
Applications of Novolic resins
Novolac resins have excellent properties of the adhesive and binding strength. So, these are used for barks lemmings making sandpapers, etc.
Also, novolac resins with 15~20% alcohol( usually methanol or butanol) are used to produce resins.
In varnishes, electrical insulation, and other protective coatings. These resins are also in the manufacture of plywood
Ion exchange resins
Novalac resins are also used in the production of ion exchange resins. These resins are useful in softening the water for textile use.
Application of resol type resins
Resol-type resins are caustic resins in which filler is not used. Caustic resins may be cured slowly by heating at 70~800C for 2~8 days. Also, they can be cured rapidly by heating with a little acid. in this way, it takes 1~6 hours for curing. the specific gravity is 1.3, tensile strength 3000~10000 lb/ inch2and have bonding strength 7000~ 15000 lb/inch2.
The resins having an amino group (-NH2-) in their structure are called amino resins. Moreover, they have been divided into two classes.
1-Urea formaldehyde resins: these are produced by the condensation of urea and formaldehyde
Hence, urea= NH2-CO-NH2 formaldehyde HCOH
2-Melamine formaldehyde resins: These are produced by the condensation product of melamine with formaldehyde.
These are carried out in two separate operations
Firstly operation involves the formation of low molecular weight, fusible and soluble resins, and the second operation involves a curing reaction which leads to cross-linked products. So various types of urea formaldehyde are produced commercially and are classified into
- Unmodified resins
- Modified resins
These are used in the preparation of molding composition and adhesives
Preparation urea-formaldehyde resins
These resins are prepared by following reactions
Formalin is made slightly alkaline by the addition of NaOH solution being PH value 8~8.5and then Urea is added to a molar ratio Urea: HCHO,1:2
The resulting solution is boiled and referred for 15 minutes acidified with formic acid and then further boiled for 5~20 minutes. The product is neutralized with NaOH and then evaporated under reduced pressure until a solid content (70%) is reached.
Formalin (2) + urea (1) → 70% solid mass + lubricant+ stabilizer+pigment(colour)
NH2-CO-NH2+ CH2O→ NH2-CO-NH-CH2-OH (methylene urea)
NH2-CO-NH-CH2-OH+ CH2O→ NH2-CH2-OH — CO — NH2-CH2-OH(Dimethylal urea)
A– when dimethyl-ol urea is further condensed with other molecules of urea forming a condensation polymer.
B– the Linear polymer formed can condense with more formaldehyde molecules to form the cross-linked products.
C- the dimethyl-ol urea molecules can condensate with each other to form linear polymers
This gives rise to a complex mixture of linear and crosslinked condensation polymers known as urea-formaldehyde resins depending upon reaction conditions and resins of the different types obtained.
Unmodified resins are not suitable for use in the surface coating because they are invaluable in common solvents ( alcohol ether etc) and do not interact readily with other resins. These limitations are overcome when the resins are modified.
Normally alcohol (normal butane) is mostly used for this purpose. The PH value is adjusted to 8 and urea is added to a given molar ratio
The resulting solution is boiled for 1 hour under a reflux condenser. Butanol (1.5 ~20 mol per of urea) is added with a little xylene water removed and an acid usually phosphoric acid as a catalyst is added and heating is continued. The solution is naturalized and concentrated to a desired solid content.
Urea (1 mol)+formaline (2.5 mol)→resultant
(PH=8, boiling under reflux butanol 1.5 ~20 mol per mol of urea, little xylene)
Resultant= a solid content of modified resins
Properties of urea-formaldehyde resins
- These resins are available in a wide range of colors.
- It is a rigid and insoluble material.
- Its molding is poor heat resistant and discolors and degrades at 800C.
- Urea formaldehyde molding has good insulating properties. Its polymer are very resistant to organic reagents, but they are attached easily by acids and alkalies
- It shows relatively high moisture absorption as compared to phenolic resins, and melamine resins.
- Specific gravity 1.5 gram/cm3
- Tensile strength 7500~11500 lb/ inch2
- Breaking strength 11000~ 17000 lb/ inch2
Application of urea-formaldehyde resins
It is used in making molding powder, as an adhesive in plywood and chipboard, used as eliminated plastics, for surface coating, and for crease-resistant fibers.
In textile finishing, it is widely used, in the paper industry and its miscellaneous uses are in the electrical industry, automobiles and in household articles.
Melamine formaldehyde resin
Melamine formaldehyde resins are carried out in operations
The first operation involves the formation of low molecular weight fusible and soluble resins and the second operation involves a curing reaction which leads to cross-linked products. Various types of melamine formaldehyde are produced commercially and are classified into
- Unmodified resins
- Modified resins
These resins are normally used in the preparation of molding composition and adhesive.
Preparation of melamine formaldehyde resins
These resins are prepared with the help of the following reactions
Let’s say trimethyl-ol melamine is R
Hence, after the condensation sheet formation takes place as follows
Resin ( A stage) + Filler(cellulose)+catalyst (TiO2,titanium dioxide) +lubricant( steric acid) → mixed over hot roller → sheet formation take place
In this stage, the sheet is grounded and dehydrated (also, removal of water takes place)
R -NH-CH2OH +HN-R-CH2OH → -H2O + RNH-CH2—CNR-CH2OH
After this in stage C cross-linking takes place.
These resins are prepared by the following reactions
Melamine + formalin → solution is derived to 80%solid content under reduced pressure (required PH 8~9, 700C, methanol or butanol
Hence, the solid content is known as melamine formaldehyde resins.
Properties of melamine resins
Melamine resins are also called molding powder or bakelite plastic or urea. The main properties are that these resins show surface hardness and are produced in a wide range of colors when melamine resins are linked together they form an infusible and insoluble coating. Melamine resins are very resistant to organic reagents but they are affected by alkalies and acids, as compared to formaldehyde resins the effect of alkalies and acids is much less
Applications of melamine resins
These are used for surface coating, These are also used as binding material with epoxy, paints and varnish materials made for the protection of the surface.
The polymer which is known for its binding properties is used in adhesives such as Araldite.
Preparation of epoxy resins
Epoxy resins are prepared by condensation of excess chloro-epoxy alkaline with dihydric phenol (diphenyl-ol propane) in the presence of alkalies or bases i.e. NaOH.Also, an excess of epichlorohydrin is used to leave the epoxy group on each end of the low molecular weight polymer 900~3000).
So the polymer is produced cross-linked by heating with a certain amine cross-linking chain. Another hydroxyl-containing compound hydroquinone and resornet, glycols, and glycerol can be replaced by di-phenyl-Propane.
Urethanes polymer contains group -NHCOO- and is formed during the reaction of di-isocyanate and glycol.
(-CNO cyanate, cyanite CN)
1-4 butane glycol+ toluene di-isocynate
Catalysts based on toluene amine are added to achieve rapid production of foam. Cross-linking takes place to give a thermosetting polymer. The basic intermediate is bifunctional foam is desired and polyfunctional groups are required if rigid foams are to be made i.e. tri-isocyanate and tri-OH group is used.
Poly urethanes can be spun into elastic fibers such as “ spandex“. Also, poly urea foams are used in the construction of interior decoration of buildings.
These are polymers derived from alcohols and acid anhydride. Also, these are called glyptic resins. Moreover, It is obtained by heating phthalic anhydride and glycerol at 1800C
Also, they can be dissolved in a suitable solvent and applied as a surface coating. On heating, the solvent evaporates, and a hard smooth protective surface results, owing to cross-linking the chain. Alkyd is normally used as a binding design and in alkyd paints.